The invention relates to refrigeration. More particularly, the invention relates to electrically-powered refrigerated transport systems.
A transport refrigeration system used to control enclosed areas, such as the box used on trucks, trailers, containers, or similar intermodal units, functions by absorbing heat from the enclosed area and releasing heat outside of the box into the environment. A number of transport refrigeration units, including units currently sold by assignee, employ a reciprocating compressor to pressurize refrigerant to enable the removal of heat from the box. Reciprocal compressors used in such applications commonly include a suction inlet and a discharge which are connected, respectively, to the evaporator and condenser of the transport refrigeration system. It is axiomatic that in order to ensure the reliability of the reciprocating compressor, the compressor should operate within the limits of the suction and discharge pressures for which it was designed. The ranges and ratios of suction and discharge pressures designed to be handled by a reciprocating compressor at various stages of operation is known as an operating envelope. The failure to operate within the compressor operating envelope will result in unnecessary wear and tear, and ultimately will bring about the premature failure of the compressor, thus creating unacceptable costs of money and time to the operator.
Transport refrigeration systems currently employ a variety of controls to manipulate the operating envelope of a reciprocating compressor. As can be shown by U.S. Pat. Nos. 5,626,027 and 5,577,390, both assigned to the assignee of the present invention, compressors can be operated in a multi-stage mode or in single stage modes depending upon operating temperature. Such disclosures further discuss generally the use of suction modulation for capacity control.
U.S. Pat. No. 6,301,911 discloses use of a controller within the transport refrigeration system which closes the suction modulation valve (the “SMV”) and/or unloads compressor cylinder banks in response to discharge pressures in excess of a preselected limit calculated or compared to a set value by the controller. An unloader, when energized has the effect of removing one or more cylinders from operation within compressor, thus effectively reducing the displaced volume within the compressor. Maintaining the reciprocating compressor within its design operating envelope may be achieved by selectively unloading compressor cylinder banks in the event that suction pressure is below the minimum suction pressure allowed, as determined the controller.
There are many operational considerations for the units. Several considerations involve the temperature at which the enclosed area is to be kept. A given unit configuration may be made manufactured for multiple operators with different needs. Broadly, the temperature may be separated into two fields: frozen goods; and non-frozen perishables. An exemplary frozen goods temperature is about −10° F. or less an exemplary non-frozen perishable temperature is 34-38° F. Many operators will have the need to, at different times, use a given unit for transport of frozen goods and non-frozen perishables. The operator will predetermine appropriate temperature for each of the two modes. prior to a trip or series, the driver will enter the appropriate one of the two temperatures.